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Publications (10 of 24) Show all publications
Larsson, F., Keller, J., Primetzhofer, D., Riekehr, L., Edoff, M. & Törndahl, T. (2019). Atomic layer deposition of amorphous tin-gallium oxide films. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, 37(3), Article ID 030906.
Open this publication in new window or tab >>Atomic layer deposition of amorphous tin-gallium oxide films
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2019 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 37, no 3, article id 030906Article in journal (Refereed) Published
Abstract [en]

A wide range of applications benefit from transparent semiconducting oxides with tunable electronic properties, for example, electron transport layers in solar cell devices, where the electron affinity is a key parameter. Presently, a few different ternary oxides are used for this purpose, but the attainable electron affinity range is typically limited. In this study, the authors develop a low-temperature atomic layer deposition (ALD) process to grow amorphous Sn1-xGaxOy thin films from dimethylamino-metal complexes and water. This oxide is predicted to provide a wide selection of possible electron affinity values, from around 3 eV for pure Ga2O3 to 4.5 eV for pure SnO2. The ALD process is evaluated for deposition temperatures in the range of 105-195 degrees C by in situ quartz crystal microbalance and with ex situ film characterization. The growth exhibits an ideal-like behavior at 175 degrees C, where the film composition can be predicted by a simple rule of mixture. Depending on film composition, the growth per cycle varies in the range of 0.6-0.8 angstrom at this temperature. Furthermore, the film composition for a given process appears insensitive to the deposition temperature. From material characterization, it is shown that the deposited films are highly resistive, fully amorphous, and homogeneous, with moderate levels of impurities (carbon, nitrogen, and hydrogen). By tailoring the metal cation ratio in films grown at 175 degrees C, the optical bandgap can be varied in the range from 2.7 eV for SnO2 to above 4.2 eV for Ga2O3. The bandgap also varies significantly as a function of deposition temperature. This control of properties indicates that Sn1-xGaxOy is a promising candidate for an electron transport layer material in a wide electron affinity range. Published by the AVS.

Place, publisher, year, edition, pages
A V S AMER INST PHYSICS, 2019
National Category
Condensed Matter Physics Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-390540 (URN)10.1116/1.5092877 (DOI)000472182400033 ()
Funder
Swedish Energy Agency, 2017-004796Swedish Research Council, 2017-00646 9Swedish Foundation for Strategic Research , RIF14-0053
Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-12Bibliographically approved
Keller, J., Bilousov, O., Wallin, E., Lundberg, O., Neerken, J., Heise, S., . . . Platzer Björkman, C. (2019). Effect of Cu content on post‐sulfurization of Cu(In,Ga)Se 2 films and corresponding solar cell performance. Physica Status Solidi (a) applications and materials science
Open this publication in new window or tab >>Effect of Cu content on post‐sulfurization of Cu(In,Ga)Se 2 films and corresponding solar cell performance
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2019 (English)In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319Article in journal (Refereed) Published
Abstract [en]

This contribution studies the effect of the initial copper content of co‐evaporated Cu(In1‐x,Gax)Se2 (CIGS) absorber films on the impact of a post‐annealing step in elemental sulfur atmosphere. The Cu concentration is varied over a wide range ([Cu]/[III] = CGI = 0.57–1.23), allowing to identify composition‐dependent trends in phase formation, chemical rearrangements and solar cell performance after sulfurization. For all samples, a ternary CuInS2 layer forms at the surface. In addition, sulfur i) is incorporated in randomly‐distributed CuIn(S,Se)2 mixed crystals underneath the CuInS2, ii) diffuses into multi‐dimensional defects (e.g. dislocations and grain boundaries) and iii) is bound in Na‐In‐S surface plates. It is found that Cu‐poor absorber composition (CGI ≤ 0.82) favors the CuInS2 growth as compared to close‐stoichiometric CIGS films, driven by a faster diffusion of Cu towards the surface. For Cu‐rich absorbers (CGI > 1) the Se‐S exchange is significantly accelerated, presumably by the presence of Cu2‐x Se phases reacting to Cu2‐xS and eventually catalyzing CuInS2 formation. Finally, open circuit voltage (VOC), fill factor (FF) and efficiency (η) of corresponding solar cells are increasing after sulfurization with increasing CGI until stoichiometry is reached. The result is explained by a mitigated Cu‐depletion of the absorber bulk after sulfurization for close‐stoichiometric CIGS.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-393049 (URN)10.1002/pssa.201900472 (DOI)
Funder
Swedish Foundation for Strategic Research , RMA15-0030
Available from: 2019-09-13 Created: 2019-09-13 Last updated: 2019-09-13
Ledinek, D., Keller, J., Hägglund, C., Chen, W.-C. & Edoff, M. (2019). Effect of NaF pre-cursor on alumina and hafnia rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells. Thin Solid Films, 683, 156-164
Open this publication in new window or tab >>Effect of NaF pre-cursor on alumina and hafnia rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells
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2019 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 683, p. 156-164Article in journal (Refereed) Published
Abstract [en]

In this work, we evaluate the effect of NaF layers on the properties of Al2O3 and HfO2 rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells. The 6 nm thin passivation layers were deposited by atomic layer deposition and neither intentionally opened nor nano-patterned in any extra-fabrication step. NaF layers, 7.5 or 15 nm thin, were deposited as precursors prior to CIGS absorber co-evaporation. The 215 nm thick absorbers were co-evaporated with constant evaporation rates for all elements. Directly thereafter, a 70 nm thick cadmium sulfide layer was deposited. Photoluminescence measurements indicate a strongly reduced recombination at the rear contact for all passivated samples compared to an unpassivated reference. Although the sample with Al2O3 passivation and a 15 nm NaF precursor layer luminesces by far the least of the passivated samples, solar cells made from this sample show the highest efficiency (8.6% compared with 5.6% for the reference with no passivation). The current-voltage curves of the solar cells fabricated from the sample with 7.5 nm NaF on top of the Al2O3 layer and both samples with HfO2 exhibit blocking behavior to various degrees, but a high photoluminescence response. We conclude that NaF precursor layers increase conduction through the Al2O3 layer, but also reduce its effectiveness as a passivation layer. In contrast, conduction through the HfO2 passivation layers seem to not be influenced by NaF precursor layers, and thus requires nano-patterning or thinning for conduction.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
Alkali, Alumina, Copper indium gallium diselenide, Hafnia, Passivation, Sodium fluoride, Ultra-thin
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-387716 (URN)10.1016/j.tsf.2019.05.024 (DOI)000469854700020 ()
Funder
Swedish Energy AgencySwedish Research Council, 621-2014-5599StandUp
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-08-08
Aboulfadl, H., Keller, J., Larsen, J. K., Thuvander, M., Riekehr, L., Edoff, M. & Platzer Björkman, C. (2019). Microstructural Characterization of Sulfurization Effects in Cu(In,Ga)Se-2 Thin Film Solar Cells. Paper presented at Atom Probe Tomography and Microscopy (APT and M) Conference, JUN 10-15, 2018, Gaithersburg, MD. Microscopy and Microanalysis, 25(2), 532-538
Open this publication in new window or tab >>Microstructural Characterization of Sulfurization Effects in Cu(In,Ga)Se-2 Thin Film Solar Cells
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2019 (English)In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 25, no 2, p. 532-538Article in journal (Refereed) Published
Abstract [en]

Surface sulfurization of Cu(In,Ga)Se-2 (CIGSe) absorbers is a commonly applied technique to improve the conversion efficiency of the corresponding solar cells, via increasing the bandgap towards the heterojunction. However, the resulting device performance is understood to be highly dependent on the thermodynamic stability of the chalcogenide structure at the upper region of the absorber. The present investigation provides a high-resolution chemical analysis, using energy dispersive X-ray spectrometry and laser-pulsed atom probe tomography, to determine the sulfur incorporation and chemical re-distribution in the absorber material. The post-sulfurization treatment was performed by exposing the CIGSe surface to elemental sulfur vapor for 20 min at 500 degrees C. Two distinct sulfur-rich phases were found at the surface of the absorber exhibiting a layered structure showing In-rich and Ga-rich zones, respectively. Furthermore, sulfur atoms were found to segregate at the absorber grain boundaries showing concentrations up to similar to 7 at% with traces of diffusion outwards into the grain interior.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
atom probe, Cu(In, Ga)Se-2, solar cells, surface treatment, thin films
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-385571 (URN)10.1017/S1431927619000151 (DOI)000466756600030 ()30853031 (PubMedID)
Conference
Atom Probe Tomography and Microscopy (APT and M) Conference, JUN 10-15, 2018, Gaithersburg, MD
Funder
Swedish Foundation for Strategic Research , RMA15-0030
Available from: 2019-06-17 Created: 2019-06-17 Last updated: 2019-06-17Bibliographically approved
Larsson, F., Donzel-Gargand, O., Keller, J., Edoff, M. & Törndahl, T. (2018). Atomic layer deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se-2 solar cells with KF post-deposition treatment. Solar Energy Materials and Solar Cells, 183, 8-15
Open this publication in new window or tab >>Atomic layer deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se-2 solar cells with KF post-deposition treatment
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2018 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 183, p. 8-15Article in journal (Refereed) Published
Abstract [en]

We investigate the possibility to combine Zn(O,S) buffer layers grown by atomic layer deposition (ALD) with KF post-deposition treated Cu(In,Ga)Se-2 (CIGS-KF) in solar cells. It is shown that the beneficial effect on open-circuit voltage from the post-deposition treatment is essentially independent of buffer layer material. However, a wet chemical surface treatment is required prior to ALD in order to achieve competitive fill factor values. A water rinse is sufficient to create an absorber surface similar to the one formed during a conventional CdS chemical bath deposition process. However, it is observed that CIGS-KF/Zn(O,S) devices made with water-rinsed absorbers systematically result in lower fill factor values than for the corresponding CIGS-KF/CdS references. This effect can be mitigated by decreasing the H2S:H2O precursor ratio during ALD initiation, indicating that the fill factor limitation is linked to the initial Zn(O,S) growth on the modified CIGS-KF surface. The best CIGS-KF/Zn (O,S) devices were fabricated by etching away the KF-modified surface layer prior to ALD, followed by a low temperature anneal. The thermal treatment step is needed to increase the open-circuit voltage close to the value of the CdS devices. The results presented in this contribution indicate that the main beneficial effects from KFPDT in our devices are neither associated with the CdS CBD process nor due to the formation of a K-In-Serich phase on the CIGS surface.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
CIGS, KF-PDT, Zinc oxysulfide, Buffer layers, Interfaces
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-358259 (URN)10.1016/j.solmat.2018.03.045 (DOI)000435624400002 ()
Funder
Swedish Energy Agency, 2017-004796
Available from: 2018-08-27 Created: 2018-08-27 Last updated: 2018-08-27Bibliographically approved
Keller, J., Chen, W.-C., Riekehr, L., Kubart, T., Törndahl, T. & Edoff, M. (2018). Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact. Progress in Photovoltaics, 26(10), 846-858
Open this publication in new window or tab >>Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact
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2018 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 26, no 10, p. 846-858Article in journal (Refereed) Published
Abstract [en]

Hydrogen‐doped In2O3 (IOH) films are used as a transparent back contact in bifacial Cu(In,Ga)Se2 (CIGS) solar cells. The effect of the IOH thickness and the impact of the sodium incorporation technique on the photovoltaic parameters are studied, and clear correlations are observed. It is shown that a loss in short circuit current density (JSC) is the major limitation at back side illumination. The introduction of a thin Al2O3 layer on top of the IOH significantly increases the collection efficiency (ϕ(x)) for electrons generated close to the back contact. In this way, the JSC loss can be mitigated to only ~ 25% as compared with front side illumination. The Al2O3 film potentially reduces the interface defect density or, alternatively, creates a field effect passivation. In addition, it prevents the excessive formation of Ga2O3 at the CIGS/IOH interface, which is found otherwise when a NaF layer is added before absorber deposition. Consequently, detrimental redistributions in Ga and In close to the back contact can be avoided. Finally, a bifacial CIGS solar cell with an efficiency (η) of η = 11.0% at front and η = 6.0% at back side illumination could be processed. The large potential for further improvements is discussed.

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-363288 (URN)10.1002/pip.3025 (DOI)000443696500008 ()
Funder
Swedish Energy Agency, 2016-008376Swedish Foundation for Strategic Research , RMA15-0030
Available from: 2018-10-16 Created: 2018-10-16 Last updated: 2018-11-02Bibliographically approved
Donzel-Gargand, O., Thersleff, T., Keller, J., Törndahl, T., Larsson, F., Wallin, E., . . . Edoff, M. (2018). Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers. Progress in Photovoltaics, 26(9), 730-739
Open this publication in new window or tab >>Deep surface Cu depletion induced by K in high-efficiency Cu(In,Ga)Se2 solar cell absorbers
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2018 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 26, no 9, p. 730-739Article in journal (Refereed) Published
Abstract [en]

In this work, we used K‐rich glass substrates to provide potassium during the coevaporation of Cu(In,Ga)Se2 (CIGS) absorber layers. Subsequently, we applied a postdeposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell effi- ciencies beyond 17%, and the addition of a PDT pushed it beyond 18%. The major finding of this work is the observation of discontinuous 100‐ to 200‐nm‐deep Cu‐ depleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the forma- tion of these patches. A second finding concerns the composition of the Cu‐depleted areas, where an anticorrelation between Cu and both In and K was measured using scanning transmission electron microscopy. Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element interdiffusion. Finally, no Cd in‐diffusion to the CIGS layer could be detected. This indicates that if CdCu substitution occurs, either their concentration is below our instrumental detection limit or its presence is contained within the first 6 nm from the CdS/CIGS interface.

Keywords
CIGS, Cu depletion, EELS, OVC, Raman, solar cell, TEM
National Category
Other Materials Engineering Energy Systems
Identifiers
urn:nbn:se:uu:diva-357120 (URN)10.1002/pip.3010 (DOI)000442501000004 ()
Funder
Swedish Research CouncilSwedish Energy AgencyEU, Horizon 2020, 720887
Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2018-11-13Bibliographically approved
Ledinek, D., Donzel-Gargand, O., Sköld, M., Keller, J. & Edoff, M. (2018). Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers. Solar Energy Materials and Solar Cells, 187(1), 160-169
Open this publication in new window or tab >>Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers
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2018 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 187, no 1, p. 160-169Article in journal (Refereed) Published
Abstract [en]

In this work, rear-contact passivated Cu(In,Ga)Se2 (CIGS) solar cells were fabricated without any intentional contact openings between the CIGS and Mo layers. The investigated samples were either Na free or one of two Na supply methods was used, i) a NaF precursor on top of the Al2O3 rear passivation layer or ii) an in situ post- deposition treatment with NaF after co-evaporation of the CIGS layer. The thickness of the ALD-Al2O3 passi- vation layer was also varied in order to find an optimal combination of Na supply and passivation layer thickness. Our results from electrical characterization show remarkably different solar cell behavior for different Na supplies. For up to 1nm thick Al2O3 layers an electronically good contact could be confirmed independently of Na deposition method and content. When the Al2O3 thickness exceeded 1 nm, the current was blocked on all samples except on the samples with the NaF precursor. On these samples the current was not blocked up to an Al2O3 layer thickness of about 6 nm, the maximum thickness we could achieve without the CIGS peeling off the Al2O3 layer. Transmission electron microscopy reveals a porous passivation layer for the samples with a NaF precursor. An analysis of the dependence of the open circuit voltage on temperature (JVT) indicates that a thicker NaF precursor layer lowers the height of the hole barrier at the rear contact for the passivated cells. This energy barrier is also lower for the passivated sample, compared to an unpassivated sample, when both samples have been post-deposition treated.

Keywords
Alkali, Back contact, CIGS, Passivation, Thin films, Rear contact, Tunneling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Energy Systems Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-357123 (URN)10.1016/j.solmat.2018.07.017 (DOI)000445308300019 ()
Funder
Swedish Research Council, 43523-1StandUpEU, Horizon 2020, 720887
Available from: 2018-08-12 Created: 2018-08-12 Last updated: 2019-08-08Bibliographically approved
Keller, J., Chalvet, F., Joel, J., Aijaz, A., Kubart, T., Riekehr, L., . . . Törndahl, T. (2018). Effect of KF absorber treatment on the functionality of different transparent conductive oxide layers in CIGSe solar cells. Progress in Photovoltaics, 26(1), 13-23
Open this publication in new window or tab >>Effect of KF absorber treatment on the functionality of different transparent conductive oxide layers in CIGSe solar cells
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2018 (English)In: Progress in Photovoltaics, ISSN 1062-7995, E-ISSN 1099-159X, Vol. 26, no 1, p. 13-23Article in journal (Refereed) Published
Abstract [en]

This contribution studies the impact of the KF-induced Cu(In,Ga)Se2 (CIGSe) absorber modification on the suitability of different transparent conductive oxide (TCO) layers in solar cells. The TCO material was varied between ZnO:Al (AZO), ZnO:B (BZO), and In2O3:H (IOH). It is shown that the thermal stress needed for optimized TCO properties can establish a transport barrier for charge carriers, which results in severe losses in fill factor (FF) for temperatures >150°C. The FF losses are accompanied by a reduction in open circuit voltage (Voc) that might originate from a decreased apparent doping density (Nd,app) after annealing. Thermally activated redistributions of K and Na in the vicinity of the CdS/(Cu,K)-In-Se interface are suggested to be the reason for the observed degradation in solar cell performance. The highest efficiency was measured for a solar cell where the absorber surface modification was removed and a BZO TCO layer was deposited at a temperature of 165°C. The presented results highlight the importance of well-designed TCO and buffer layer processes for CIGSe solar cells when a KF post deposition treatment (KF-PDT) was applied.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-332827 (URN)10.1002/pip.2925 (DOI)000418097200002 ()
Funder
Swedish Energy Agency, 2012-004591
Available from: 2017-11-02 Created: 2017-11-02 Last updated: 2018-01-17Bibliographically approved
Larsen, J. K., Keller, J., Lundberg, O., Jarmar, T., Riekehr, L., Scragg, J. J. & Platzer Björkman, C. (2018). Sulfurization of Co-Evaporated Cu(In,Ga)Se-2 as a Postdeposition Treatment. IEEE Journal of Photovoltaics, 8(2), 604-610
Open this publication in new window or tab >>Sulfurization of Co-Evaporated Cu(In,Ga)Se-2 as a Postdeposition Treatment
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2018 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 8, no 2, p. 604-610Article in journal (Refereed) Published
Abstract [en]

It is investigated if the performance of Cu(In,Ga)Se-2 (CIGSe) solar cells produced by co-evaporation can be improved by surface sulfurization in a postdeposition treatment. The expected benefit would be the formation of a sulfur/selenium gradient resulting in reduced interface recombination and increased open-circuit voltage. In the conditions used here it was, however, found that the reaction of the CIGSe layer in a sulfur environment results in the formation of a CuInS2 (CIS) surface phase containing no or very little selenium and gallium. At the same time, a significant pile up of gallium was observed at the CIGSe/CIS boundary. This surface structure was formed for a wide range of annealing conditions investigated in this paper. Increasing the temperature or extending the time of the dwell stage had a similar effect on the material. The gallium enrichment and CIS surface layer widens the surface bandgap and therefore increases the open-circuit voltage. At the same time, the fill factor is reduced, since the interface layer acts as an electron barrier. Due to the balance of these effects, the conversion efficiency could not be improved.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018
Keywords
Alloying, Cu(In, Ga)Se-2 (CIGSe), postdeposition treatment, surface treatment, thin-film solar cells
National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-348836 (URN)10.1109/JPHOTOV.2018.2793759 (DOI)000425525100034 ()
Funder
Swedish Foundation for Strategic Research , RMA15-0030
Available from: 2018-04-23 Created: 2018-04-23 Last updated: 2018-09-14Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-3461-6036

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